1. Field of the Invention
This invention relates to an ultraviolet crystal filter material for creating a solar blind optical filter and a UV detection system having an ultraviolet crystal filter.
2. Discussion of Related Art
Ultraviolet (UV) filters and sensors, are used for a variety of applications, including fire detection systems, electrical corona detection systems, aircraft landing aids, and missile approach warning systems. Due to the small amount of solar radiation in the UV range (i.e., wavelengths of about 350 nm and below) at low altitudes, it is possible to have UV detection in daylight conditions. UV sensors that operate in a wavelength range of between about 350 nm to about 200 nm are considered to be xe2x80x9cSolar Blindxe2x80x9d.
One example of a solar blind UV detection system occurs in fire alarm systems where the presence of a threshold amount of UV radiation is used to detect a fire. Upon detection of the fire, the alarm system can trigger fire alarms, fire suppression systems or calls to the local fire department.
Another example use of a solar blind UV sensor can be found in the AN/AAR-47 missile warning system. This system is typically used on helicopter platforms and utilizes UV sensors to detect infrared or radar guided missile threats. If a missile threat is detected, appropriate countermeasures, such as the ejection of flare decoys or chaff, can be deployed.
A UV optical system includes optical components for focusing radiation onto a UV sensitive sensor, a filter for reducing the amount of radiation not in the UV window (e.g., 200-350 nm), and electronics which process and act on signals from the UV sensitive sensor. To date, most UV optical filters include a series of optical filter elements that are stacked to achieve a desired spectral response. For example, a solar blind UV filter may have a pass band of between the wavelengths of about 200 nm to about 350 nm. Each optical element in the series is a single separate optical filter.
FIG. 1 shows an optical filter 1 having a single filter element, film 2, on a substrate 3. Substrate 3 can be an optically transparent layer that provides structural strength. An optically transparent layer is a layer of material transparent in the wavelength range of interest, e.g. between about 200 nm and about 350 nm for a solar blind UV filter. Alternatively, substrate 3 may be a detector or other optical element related to detection of the transmitted optical radiation. Also, film 2 may be mounted alone or stacked with other films 2 that also provides optical filtering having a particular spectral response. Typically, film 2 is of a commercially available organic dye suspended in polyvinyl alcohol plastic.
There are numerous problems with the use of organic dye films. These films degrade with exposure to fluids and temperatures above 100xc2x0 C. Additionally, organic dye films typically have low transmission in the desired band-pass region. Organic dye films are also physically easily damaged. Examples of organic dyes available for films can be found in M. Kasha, Opt. Soc. Am., 38, 929, (1948), and S. F. Pellicori et al., Apl. Oct. 5, 12, 1966.
It is desirable, therefore, to provide a UV filter with better transmission characteristics within the desired band-pass region. It is also desirable for the filter to be stable at temperatures as high as 150xc2x0 C. Also, it is desirable for the filters to be chemically stable, especially with respect to fluids that may be present in the working environment or during manufacturing of the filter-detector system. Additionally, it is desirable to provide filters that have low levels of fluorescence within the operating band of the filter when exposed to UV radiation.
In accordance with the present invention, a crystal filter having absorption bands such that a transmission pass band is formed is presented. In most embodiments, the pass band is appropriate for use as a solar-blind UV filter (i.e., around 200 nm to around 350 nm wavelength). The crystal filter is a single-crystal host material doped with one or more optically active (e.g., optically absorbing) dopant materials. The single-crystal host material is substantially optically transparent (i.e., having low absorption) at least within the desired pass band. The optically active dopants create optical absorption bands above and below the desired pass band. In the UV filter applications, the single-crystal host material can be any single crystal that is transparent in the active region (e.g., wavelength region between about 200 to about 350 nm for solar-blind UV filters), including the alkaline earth fluorides, zinc fluoride or cadmium fluoride. The single-crystal host material can be codoped with a lanthanide or actinide fluoride and a corresponding lanthanide or actinide nitride, boride, oxide, carbide or hydroxide material. Any combination of codopants that provide the desired absorption pass-band can be utilized.
The alkaline earth fluoride host materials include magnesium fluoride (MgF2), calcium fluoride (CaF2), strontium fluoride (SrF2), and barium fluoride (BaF2). Additional fluoride host materials include ZnF2 and CdF2. Each of these host crystal materials have suitable optical transmission in the wavelength region of about 200 nm to about 350 nm and can readily be codoped with lanthanide or actinide fluorides and lanthanide or actinide nitrides, borides, oxides, carbides or hydroxides to provide the desired spectral characteristics. The lanthanide and actinide series of elements includes Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and U. The doping is accomplished by addition of the appropriate fluorides and nitride, boride, oxide, carbide or hydroxide to a mixture that forms the crystal melt during crystal growth.
A particular band-pass region for the filter can be arranged by appropriately doping the host crystal. In one embodiment, denoted filter material type 1 (FMT-1), CeF3 and CeN are added to CaF2 to yield a crystal filter which has a wide absorption band between about 290 nm to about 335 nm, centered around about 310 nm. This codoping enables the crystal filter to absorb in a sufficiently wide band to block any unwanted fluorescence near the operating UV wavelengths of the detector, while allowing transmissions in a portion of the 200-350 nm wavelength range, about 220 nm to about 280 nm.
In another embodiment of the invention, denoted FMT-2, the absorption band is widened to include higher wavelengths by addition of CeF3, CeN, EuF3, and EuN to CaF2. The FMT-2 crystal can have an absorption band which extends from about 290 nm to 410 nm while having good transmission in the desired UV wavelengths of between about 220 nm and about 280 nm.
Yet another embodiment of the invention, denoted FMT-3, includes a calcium fluoride (CaF2) host crystal codoped with cerium fluoride (CeF3) and cerium carbide (CeC2). A fourth embodiment, denoted FMT-4, includes a calcium fluoride (CaF2) host crystal codoped with cerium fluoride (CeF3) and cerium hydroxide (Ce(OH)2).
In some embodiments of crystal filters according to the present invention, the crystal filter includes a single crystal fluoride host (e.g., MgF2, CaF2, SrF2, BaF2, ZnF2 or CdF2) codoped with a lanthanide or actinide fluoride compound and a lanthanide or actinide nitride, boride, carbide, oxide or hydroxide compound. The dopant fluoride compound does not necessarily include the identical lanthanide or actinide element as the dopant nitride, boride, carbide, oxide or hydroxide compound. Additionally, the crystal filter may be codoped with more than two codopants.
Crystal filters according to the present invention are readily grown using a number of well-known crystal growth techniques, including Czochralski and Bridgeman techniques. The particular crystal growth condition for growing crystal filters are further discussed below. In most embodiments, alkali fluorides are added to the crystal growth to increase distribution coefficients of the dopants. The alkali fluorides include Li, Na, K, Rb, and Cs fluoride. In some growth methods involving nitride, oxide or hydroxide codopants, the corresponding gaseous component can be introduced to the crystal melt during growth or diffused into a pre-grown crystal in order to form the nitride, oxide or hydroxide codopant.
Crystal filters according to the present invention are stable in air to high temperatures. The FMT-1, FMT-2, FMT-3 and FMT-4 crystals, for example, are stable to at least about 450xc2x0 C. The crystal filters are also very durable and can be easily fabricated into optical elements for use in UV detection systems.
In accordance with another aspect of the invention, a solar-blind UV detection system having a crystal filter is provided. The solar-blind UV detection system includes an optical array, a crystal filter, and a UV detector. In most embodiments, the crystal filter transmits in the range of about 200-350 nm range while absorbing radiation in ranges above about 350 nm in order to block solar radiation from the UV detector system. The filter is a single crystal, codoped filter as has been described above. Other embodiments of the invention can have pass bands in other than the 200-350 nm range.